Plutons? Brown dwarfs? A new crowd in the solar system.

ByPeter N. SpottsJeffrey WhiteAugust 17, 2006

BOSTON AND PRAGUE, CZECH REPUBLIC
— With the stroke of some pens, the number of planets in the solar system could expand next week.

Instead of the classic nine, the new system would have at least 12. Rather than the tidy division of four rocky planets, four giant gas balls, and one icy blob, the new arrangement would establish a new class of planets – plutons, according to a proposal due to be presented Wednesday at an international astronomers meeting in the Czech Republic. It would also embrace some of the largest asteroids, which would become planets. To add a dash of "huh?," anything smaller than Mercury would become "small solar-system bodies."

It's the plutons – Pluto-type objects – that have the potential to do the most to bulk up the solar system. The astronomers are scheduled to vote on the plan Aug. 24.

In the end, the change "might be OK," says Caltech astronomer Michael Brown, who dubs it the "no ice ball left behind" approach. He estimates the solar system might end up with some 53 planets as astronomers continue to explore the solar system's outer reaches. The plan provides details "only a scientist would love," he adds, "while other people will look and say, 'That's crazy,' then go about their business."

Another perspective: "Even though scientists are going about their regular scientific business and discussion, they do realize that the question of what is a planet is something that has a large popular interest," says Dr. Richard Binzel, an astronomy professor at the Massachusetts Institute of Technology, and one of the people who helped draft the new definition.

The impetus for the expansion: astronomers' first attempt to define a planet. In short, it would be anything with enough gravity to gather itself into a rounded shape (and orbiting a star).

Strange as it may seem, astronomers have never worked with a formal definition of "planet." Instead, they've used a "we'll know one when we see one" approach. But that idea began to break down as scientific debate heated up over Pluto.

Initially Pluto earned "planet" status when US astronomer Clyde Tombaugh discovered it in 1930. Since then, as astronomers learned more about the range of objects the solar system contains, many of them eased the icy ball into different, sometimes overlapping categories – from a giant comet to a "minor planet" to a member of an extended population of icy objects beyond Neptune, most of which are gathered in what has become called the Kuiper Belt.

Last year, Dr. Brown and colleagues applied the equivalent of a cattle prod to the discussion when they published their discovery of an object dubbed 2003 UB313. It's roughly half again as far from the sun as Pluto's average orbit of 39 astronomical units (one A.U. is 93 million miles). And it's slightly larger than Pluto. So, is 2003 UB313 a planet? If not, then Pluto can't be one either.

Enter six astronomers and author Dava Sobel, tapped by the International Astronomical Union to find a way out of this classification conundrum. After two years of work, intense debate, and a final night where "several members admitted they had not slept well," the seven reached unanimous agreement on the proposal, according to panel chairman Owen Gingerich, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

So what would gain the "planet" label? The asteroid Ceres, initially thought to be the eighth planet when it was discovered in 1801, shifts back to planet status. Moreover, any "planet" taking longer than 200 years to orbit the sun would be a "pluton," a new official category of planets. Under this definition, Pluto and its "moon" Charon become two planets because of their similar sizes. They would also represent the solar system's first binary planets. And 2003 UB313 would join the club as well, bringing the total to 12 planets.

Even if astronomers OK the plan next week, they still face the daunting task of defining a planet at the other end of the size scale, as well as outside the solar system.

This debate, however, is less emotionally charged, Brown acknowledges. There's far less sentimental attachment to planets that dwarf Jupiter orbiting other stars than the orbs humans have tracked and mythologized for centuries.

Still, it hangs like a loose thread: When is a monster gas ball a planet and when is it a brown dwarf, a star wannabe that never grew massive enough to raise the wick on its nuclear lantern? Brown dwarfs are less than 75 times the mass of Jupiter. Brown notes that extra-solar planets reach as much as 20 times the mass of Jupiter. But researchers have tended to settle on 13 Jupiter masses as the upper limit – a kind of cutoff point below which the object is incapable of burning a heavy form of hydrogen called deuterium.

Brown says he'd prefer a definition that would give the object's formation more weight. Stars, for example, form from collapsing blobs of interstellar gas and dust. They then gather additional material around them in a disk. Even the biggest "exoplanets" appear to have formed this way, he says.